What is a photocoupler?

It is an element capable of electrically insulating an input signal from an output signal.

A general semiconductor element has only one GND terminal , but the photocoupler has one each on the input and output side, both of which are electrically insulated inside. To insulate, an electrical signal is once converted into an optical signal inside and made into the electrical one again. In order to do this, an LED and a light receiving element are internally combined to transfer the signal by means of light.